It is known to measure the value of one or more parameters of a gas dissolved in a liquid, for example its concentration or its pressure. As an example, it is known to measure the concentration of methane or carbon dioxide in oceans or lakes, for example to determine the influence of pollution on those environments, or to determine the pressure of a gas near a pipeline in order to detect any leaks.
In order to perform such measurements, a measuring device comprises, in a known manner and as illustrated in FIG. 1, a cylindrical body 2 inside which a measuring sensor (not shown) is mounted for measuring the value of the parameter(s) to be measured.
The cylindrical body 2 comprises, at one of its ends 2a, a membrane 3 for capturing gases dissolved in a liquid L in which the device 1 is submerged, maintained on the body 2 by a maintaining element 4. Such a membrane 3 allows the gases dissolved in the liquid L to penetrate the inside of the body 2 of the device 1.
In order for the membrane 3 to be able to withstand the pressure of the liquid L and not tear, it is known to add, between the membrane 3 and the cylindrical body 2, a support element 5 for the membrane 3 assuming the form of a porous metal disc.
The maintaining element 4 is mounted bearing on a transverse inner wall 2b of the end 2a of the body 2, for example by screwing or using a plurality of screws (not shown) in order to maintain the membrane 3 and the support element 5 on the body 2. Such a maintaining element 4 ordinarily assumes the form of a ring defining a cylindrical central opening 4a, the dual function of which is to maintain the membrane 3 and the support element 5 on the end 2a of the body 2 while allowing the liquid L to come into contact with the membrane 3 via the central opening 4a. 
In order to make the device 1 tight with respect to the liquid L and thereby prevent the liquid L from penetrating the inside of the cylindrical body 2, the device 1 comprises, still in reference to FIG. 1, a first O-ring 7 arranged between the end 2a of the body 2 and the maintaining element 4 and a second O-ring 8 arranged between the membrane 3 and the maintaining element 4.
The body 2 of the device lastly comprises an inlet conduit 9a making it possible to convey the gas coming from the liquid L from the porous support element 5 to the measuring sensor and an outlet conduit 9b making it possible to convey the gas from the measuring sensor to the porous support element 5 so that it returns into the liquid L.
During the mounting of the device 1, the support element 5 is first arranged on the end 2a of the body 2, then the membrane 3 is placed on the support element 5, and lastly the maintaining element 4 is mounted on the body 2 in order to make the device 1 impermeable to the liquid L while allowing the gas dissolved in the liquid to penetrate inside the body 2.
During the operation of the device 1, the gas dissolved in the liquid L traverses the membrane 3 of the liquid L, then the porous support element 5, and is next conveyed to the measuring sensor by the inlet conduit 9a so that the sensor can determine the value of the parameter(s) of the gas to be measured. Once the measurement is done by the sensor, the gas is conveyed by the outlet conduit 9b from the sensor to the porous support element 5 and the membrane 3, which it traverses up to the liquid L.
However, in such a device, the gas exchange at the membrane 3 between the liquid L and the inlet conduit 9a is not very effective, since the exchange surface between the inlet conduit 9a and the porous support element 5 is relatively small, the surface of the opening of the inlet conduit being much smaller than the surface of the porous support element 5, which does not allow a satisfactory circulation of the gas in the device 1 and is therefore a drawback.
One obvious solution would be to increase the exchange surface, in particular by increasing the dimensions of the inlet conduit. Such an increase is not, however, satisfactory, since it increases the response time of the device, i.e., the time necessary for the sensor to measure the value of the parameter(s) of the studied gas, which is a significant drawback.